Generally, refrigeration cycles, including closed cycle refrigeration systems are well known in the art. In a typical closed cycle refrigeration system intended for household or commercial applications, the system consists of a compressor that compresses the refrigerant, which is then cooled in a condenser, then expanded through an expansion device to produce cold refrigerant which is passed through an evaporator to provide cooling effect to the device or medium to be cooled and then the warmed refrigerant is returned to the compressor.
However, when the temperatures to be achieved are in the semi-cryogenic (230° K to 120° K) or cryogenic range (below 120° K), then efficient and reliable systems are hard to find. This problem is especially critical when temperatures below 120° K, e.g. close to liquid nitrogen condensation/boiling temperature (77° K), are to be achieved. To produce refrigeration at these temperatures a refrigeration system must use very high pressure refrigerant gases such as nitrogen, argon or methane. This in turn requires more expensive equipment and frequent maintenance.
A closed cycle refrigeration system utilized to provide relatively low temperatures, for example significantly below about 230° K, in the cryogenic range, for example about 70° K, encounters certain problems in providing an efficient system. This is due to the fact that the system is expected to operate efficiently first, from ambient temperatures (about 300° K) during cooldown and then, at steady state conditions at temperatures in the range of from about 230° K down to the range of about 70° K. It is known to use a large orifice during the initial cooldown to allow a large mass flow, followed by reducing the size of the orifice opening during steady state operation at low temperatures. Both manually adjusted and automatically adjusted orifices have been used to accomplish the goal of varying the size of the opening in a throttle device.
U.S. Pat. No. 5,595,065 (the specification of which is incorporated herein by reference) discloses both the older manual adjustment of the orifice opening and means for automatic adjustment of the orifice opening during the cooldown from ambient temperature to steady state operation in the cryogenic range.
Prior art systems of the type discussed in the '065 patent, tend to generate, during cooldown from ambient temperatures, from about 40% up to almost 60% of the peak refrigeration capacity obtained at steady state operation at temperatures below 230° K down to about 70° K. Typically, such prior art systems are designed so that the refrigeration flow, and hence the capacity, is low at the start of the cooldown compared to the flow and refrigeration capacity during steady state operation at temperatures below about 230° K.
It has also been suggested in the prior art to use mixed refrigerant systems to achieve temperatures in the semi-cryogenic range by employing cascade refrigeration systems with successive phase separators to permit use of a single compressor, such as described in U.S. Pat. No. 3,768,273. This type system is not practical, however, for small refrigeration systems where heat exchangers and cold heads are to be close to or need to be integrated with the system interface to be cooled.
In another method described in U.S. Pat. No. 5,337,572 an oil lubricated rolling piston compressor is shown to be suitable for use in a single stage compressor system without using phase separators or cascade heat exchangers. Patentee describes a flammable mixed component refrigerant fluid for a system providing temperatures below 150° K.
A system using flammable mixed component refrigerant fluid to reach cryogenic temperature is also discussed in British Patent No. 1,336,892.
Neither U.S. Pat. No. 5,337,572 or British Patent No. 1,336,892 teach or suggest a system or how to design a system that provides efficient long term operation, due to the problems associated with oil circulating with the mixed component refrigerant and the water vapor that the system desorbs during such long term operation. There is no teaching by patentees or guidance as to what characteristics the fixed orifice throttle device must have to provide efficient cooling from room temperature to temperatures below about 230° K. Neither reference provides sufficient refrigeration performance by a system that will allow the device interface to cool down when the starting heat load is higher than that which the system generates at room temperature. Under these conditions a larger system will be required, thus increasing the cost of the equipment and operating costs, a significant problem in actual applications. Neither patent teaches or suggests how to utilize non-flammable mixed component refrigerant fluids in such systems.
Mixed component refrigerant fluids of the prior art require a variable restrictor device to achieve cooldown from room temperature. This in turn requires the restrictor to be set at one level for cooldown and another level for steady-state operation. This complicates the system mechanically and complicates operation. Another method of achieving higher refrigeration in prior art systems deals with modifying mixed component refrigerant fluid components such that the density ratio of the refrigerant is manipulated to achieve better flow. Thus additional refrigerant components must be added to a mixed refrigerant fluid which do not add to a refrigeration effect. The problem related to cooldown from ambient is not fully recognized and addressed in the prior art. Remedies suggested prior to the present invention end up complicating system hardware, its method of operation and/or the composition of the mixed component refrigerant.